Age-Dependent Immune Maturation Defines the Susceptibility Window of Spodoptera frugiperda to the Parasitoid Eiphosoma vitticolle

Abstract Successful deployment of parasitoids as biological control agents depends critically on the immune status of the target host. We evaluated parasitoidism by the ichneumonid endoparasitoid Eiphosoma vitticolle Cresson across all six larval instars of the fall armyworm Spodoptera frugiperda (J.E. Smith) under controlled laboratory conditions (24.5 ± 1.0°C; 76.0 ± 10.0% RH; 12:12 L:D). Despite high initial parasitoidism rates in instars I–IV (58.4–77.1%), successful adult parasitoid emergence occurred exclusively in instars I–III (26.2–33.0%), with absolute failure (0%) in instars IV–VI. This dramatic discordance revealed an ontogenetic immune barrier mediated by granulocyte-dependent hemocytic encapsulation, confirmed by a ~ 5-fold increase in circulating hemocyte density from instar I (~ 5,000 cells/µL) to instar IV (> 25,000 cells/µL). Sex ratio exhibited a strong male bias in small hosts (100% males in instar I), equilibrating to 55% males in instar III, consistent with resource-based sex allocation theory. Parasitoidism induced significant developmental delays in susceptible instars (up to 53% slower than controls), with total host mortality reaching 38.8–48.6% in early instars, including a 'partial immunity' component (20.7–29.1%) representing hosts that eliminated parasitoids but died from immune-mediated collateral damage. These findings define a six-day biological control window (instars I–III) requiring precise temporal synchronization through pheromone-trap monitoring, degree-day phenological models, and augmentative releases during early larval peaks.